1. Field of the Invention
The present invention relates to the accommodation of various optical recording media of different thicknesses by an optical pickup device. More particularly, the present invention relates to a chromatic aberration-correcting element which allows an optical pickup device to universally adopt optical discs different in thickness and in recording/reproducing laser wavelength in the presence of only a single objective lens optimally adapted for blue lasers, and an optical pickup device provided with the same.
2. Description of the Related Art
As means for storing image and/or voice information or as secondary memory units for computers, optical recording media (hereinafter referred to as just “optical media”), which are now predominant over other recording media, are largely classified into compact discs (CD) with a storage capacity of 650 MB and digital versatile discs (DVD) with a storage capacity of 4.7 GB. Determining the amount of information able to be recorded or regenerated, the recording density of optical media depends on the optical spot on which a laser can be focused through an objective lens. A focused laser beam has a focal spot diameter scaling S according to the following Equation 1:S∝λ/NA  Equation 1
where λ is the wavelength of the laser beam and NA is the objective lens numerical aperture.
Hence, an increase in recording density is mainly enabled by reducing the spot size by using shorter wavelength light and by increasing the numerical aperture of the objective lens. Nowadays, a blue-violet laser beam is used as a short wavelength light source with an NA over 0.6. For example, as much as 22 GB can be stored by using a blue wavelength (405 nm) in combination with an NA=0.85 objective lens.
However, the high numerical aperture places a stringent constraint on disc tilt because tilt causes coma aberration, which is represented by the following equation.
                              W          31                =                              -                          d              2                                ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢              sin              ⁢                                                          ⁢              θcos              ⁢                                                          ⁢              θ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                    θ                                                  )                                            5                /                2                                              ⁢                      NA            3                                              Equation        ⁢                                  ⁢        2            
where W31 is a coma aberration, θ is a tilt angle of an optical disc, n is a refractive index of the optical disc, d is a thickness of the optical disc, and NA represents a numerical aperture of the optical disc.
Usually, the thickness of an optical disc refers to that from a light incident layer to a recording layer in the optical disc. The refractive index also refers to that measured in the above thickness range. In general, because signal degradation due to disc tilt is in inverse proportion to the wavelength of the incident laser beam and in direct proportion to the cube of NA of the objective lens, the tolerance for disc tilt sharply decreases with an increase in storage density. In order to compensate for this, an optical disc with a high recording density is reduced in thickness. This is corroborated through Equation 2 in that, to attain tolerance for disc tilt, disc thickness must be reduced as the numerical aperture of the objective lens is increased for high storage density. For instance, CDs which use a 780 nm beam are 1.2 mm thick, and DVDs are reduced to a thickness of 0.6 mm due to the use of a 650 nm beam. Thus, an optical disc using a blue laser, hereinafter referred to as a ‘blue-ray disc’ (BD), is anticipated to be 0.1 mm thick. Of course, the numerical aperture of the objective lens is 0.45 for CDs and 0.6 for DVDs. In the case of BDs, the numerical aperture may be as high as 0.85. As such, an important problem anticipated to stem from the development of new standards of optical discs is found in the compatibility between the new standards and pre-existing ones.
The use of blue wavelength laser beams for recording data at high density requires a high numerical aperture, but causes a problem in that high numerical apertures amplify the influence of aberrations. Particularly the influence of wavelength fluctuation of a semiconductor laser is aggravated. The wavelength fluctuation of a semiconductor laser occurs in response to a change in the temperature of the operational environment or in the beam energy of the semiconductor laser. For instance, when the environment for the emission of a beam from a semiconductor laser changes by one degree Celsius from 25° C., the wavelength fluctuates within a range of ±0.07 nm. A change of 1 mW in the emission energy of a semiconductor laser results in a wavelength fluctuation within a range of ±0.04 nm. In an optical pickup device capable of recording and reproducing data, a semiconductor laser operates with a pulse output of 30 to 50 mW upon recording, which leads to a momentary wavelength fluctuation in the range of 1.2 to 2 nm. A momentary wavelength fluctuation of as large as 2 nm causes a focal point to deviate a distance of as long as 0.35 um when an objective lens with a numerical aperture of 0.85 is used. The positional deviation of the focus causes a change in the total focal length of the objective lens because the refractive index of the material of the objective lens changes with the wavelength. The depth of focus of an objective lens is represented by λ/(NA)2. When the positional deviation of the focus is as large as or larger than ±λ(2NA2), optical spots small enough to reproduce data cannot be formed. Where λ=410 nm and NA=0.85, if the positional deviation of focus is as large as λ/(2NA2)=0.28 um, it is almost impossible to record or reproduce data.
Thus, in order to remove chromatic aberrations caused by the use of a short wavelength such as a blue laser, an optical element for correcting chromatic aberrations is positioned in the optical path of the laser. Japanese Pat. Laid-Open Publication No. 2001-256672 discloses a chromatic aberration-correcting element which is provided in the path through which a laser beam propagates. The chromatic aberration-correcting element, consisting of a diffraction optical element and a concave lens, is provided between an objective lens and a semiconductor laser so as to correct chromatic aberration in a short wavelength such as a blue laser.
However, the chromatic aberration-correcting element is not an element that allows DVDs and CDs to be compatibly adopted. Because, after being incident on the chromatic aberration-correcting element, DVD/CD laser beams diverge or converge, the position where the chromatic aberration-correcting element can be inserted remains limited. For this reason, an optical pickup device provided with the chromatic aberration-correcting element cannot integrate a BD optical system and a DVD/CD optical system therein, but has a complicated structure.
Therefore, there is a need for a chromatic aberration-correcting element that compatibly use light sources of different wavelengths so as to integrate a BD optical system and a DVD/CD optical system therein.